1. Field of the Invention
This invention relates to a process for producing a high concentration coal-water slurry and more particularly it relates to a process for producing a stabilized, high concentraion coal-water slurry according to a multi-stage, liquid-feed process.
2. Description of the Prior Art
Recently, coal has come to be actively utilized in place of petroleum mainly at thermal power stations. However, coal as a solid fuel is difficult to handle and also the proportion of its transport cost upon the overall cost of coal is great. Thus, development of techniques for preparing a coal slurry to thereby handle it in liquid form has been energetically carried out.
One of the techniques is directed to COM (Coal and Oil Mixture) which is a mixture of heavy oil with coal. In the case of COM, however, the ratio by weight of heavy oil to coal is about 1:1; hence COM cannot be regarded as a completely oil-free fuel, and also its merit of cost is small. Further, methacoal which is a mixture of methanol with coal is so expensive that it has not yet been practically used. Whereas CWM (Coal and Water Mixture) which is a mixture of coal with water is fully practical in respect of cost, it has recently been greatly noted. For producing this CWM, there has generally been employed a process of adding water to coal and milling the mixture in a wet manner. However, a problem has been raised that if the proportion of the water content in CWM is high, its thermal efficiency at the time of combustion is reduced, while if it is low, the viscosity of CWM rises to increase the pressure loss at the time of its transportation. Further, since CWM consists of coal particles and water, an additional problem as to storage has been raised that coal particles settle with lapse of time and separate from water. In order to overcome these drawbacks, the particle size distribution of coal particles is adjusted and further an additive (dispersing agent) is added to CWM to produce a CWM having a high coal concentration, a low viscosity and a good stability. Namely, in order to produce a slurry having a high coal concentration, a low viscosity and a good stability, it has been said to be preferred to add an additive to coal and water to be fed and mill the coal to such a particle size that a maximum packing fraction can be given. However, such high concentration wet milling has drawbacks that if an additive is added all at once, a superfluous amount of the additive is adsorbed onto the coal surface to increase the amount of the additive consumed, and since additives are expensive, their addition in a large amount increases the cost of product slurry. Thus, it has become a problem to be solved to use an additive in an amount as small as possible and yet produce a stabilized, high concentraion slurry.
The object of the present invention is to provide a process to overcome the above-mentioned drawbacks of the prior art and to produce a coal-water slurry having a high coal concentration (generally 60 to 80 % by weight or more) and a lower viscosity, at a lower cost and with a smaller amount of an additive used.